Symphony adapter

[timkpaine]

Fixes #86

Needs:

• update pyproject.toml with symphony extra with requests
• normalize response of subscribe to either be a single message or list of messages between this and Slack adapter skipping for now, unclear which to prefer
• Tests
• (after merge) Docs added to https://github.com/Point72/csp/wiki/3.-Adapters
• (after Refactor and test examples #26) Step-by-step example / walkthrough

[stephenmarkacs]

Looks good. I grabbed out the text of symphony.py and compared it vs original and indeed the difference is just how the secrets are handled (here passed in) and formatting.